Biodiesel fuels composed of long chain fatty acid alkyl esters are produced from plant oils or animal fats, which are natural products, and thus, biodiesel fuels are increasingly introduced and becoming popular mainly in Europe and South East Asia, as environmentally friendly alternative fuels for light oil. European Standard EN 14214, Japanese standard JIS K2390, and American Standard ASTM D6751 are defined as standards for regulating the quality of biodiesel fuels in order to allow the safe use of biodiesel fuels as automotive fuels, and these quality standards must be met when biodiesel fuels are mixed with light oil and supplied to the market.
To improve the oxidation stability of biodiesel fuels, techniques of adding an antioxidant are known (Patent Documents 1 and 2). However, a large amount of antioxidant needs to be added in order to improve the oxidation stability of a biodiesel fuel with a high unsaturated fatty acid content, which leads to an increase in the fuel production cost. Further, the addition of an antioxidant to the biodiesel fuel does not solve the problem of sludge generation, and accordingly, further development of techniques which allow for improving the oxidation stability of biodiesel fuels is demanded.
The present inventors have therefore proposed hydrogenating catalysts capable of producing a biodiesel fuel having an extremely superior oxidation stability, by selectively hydrogenating polyunsaturated fatty acid alkyl esters containing two or more double bonds and having a poor oxidation stability, among the fatty acid alkyl esters contained in the biodiesel fuel, to monounsaturated fatty acid alkyl esters having a relatively excellent cold flow property and oxidation stability, under hydrogen pressure of atmospheric pressure (Patent Documents 3 and 4).
However, although the catalysts proposed by the present inventors have an excellent capability for selectively hydrogenating polyunsaturated fatty acid alkyl esters to monounsaturated fatty acid alkyl esters, biodiesel fuels derived from natural fats and oils contain various types of impurities such as sulfur compounds, nitrogen compounds and trace metals, and these impurities have caused a problem of poisoning the hydrogenating catalysts, thereby reducing their activity.
The removal of the impurities causing the inactivation of the catalyst is effective as a measure for solving the above mentioned problem, and a purification method utilizing an adsorbent is used for the removal (Patent Document 5). While the method utilizing an adsorbent has a high separation performance, it has an upper limit on the amount of adsorption, and thus, there are problems such as the necessity of using a large amount of adsorbent when the biodiesel fuel contains a large amount of impurities, and the dependency of the adsorption performance on the type of the adsorbent or the composition of the raw oil (crude biodiesel fuel), as well as the regeneration and disposal of the adsorbent after use.
Further, Patent Document 6 proposes a catalyst for hydrogenating fats and oils and a method for producing the catalyst. However, the techniques disclosed therein require that the hydrogenation be carried out under an extremely high pressure of 20 MPa, for example, and it is not clear as to whether these techniques are effective under low pressure conditions (1 MPa or less).
In addition, in order to carry out the hydrogenation of aromatic hydrocarbons contained in light oil using a noble metal catalyst, a method is disclosed in which 100 to 10,000 ppm of oxygen and/or an oxygen-containing organic compound is/are introduced into the reaction system along with hydrogen, to impart the catalyst with a resistance against impurities such as sulfur compounds and nitrogen compounds contained in light oil. However, the above mentioned technique requires that the hydrogenation be carried out under an extremely high pressure of 3.9 MPa, for example, and it is not clear as to whether it is effective under low pressure conditions (1 MPa or less), such as the conditions for carrying out the hydrogenation of a biodiesel fuel. Further, the above mentioned technique requires the addition of a heavy rare earth element(s) to the noble metal catalyst, in order to obtain the effect of adding oxygen and/or the oxygen-containing organic compound, and although it is effective in stabilizing the desulfurization activity, it is not effective in stabilizing the hydrogenation activity. Still further, the addition of oxygen and/or the oxygen-containing organic compound resulted in a decrease in the initial activities of both the desulfurization and the hydrogenation activities (Patent Document 7).